1. Field of the Invention
This invention relates generally to a radio receiver, and more particularly to a radio receiver which operates as a double super upon reception of a short wave signal, and as a single super upon reception of a medium wave signal.
2. Description of the Prior Art
It is well known in the art to provide a radio receiver which can receive both short and medium wave signals. When a short wave signal is received, the prior art radio receiver converts the received short wave signal into an intermediate frequency signal of 10.7 MHz, and then further converts this intermediate frequency signal into a second intermediate frequency signal of 455 KHz, and thus is referred to as a double super. On the other hand, when a medium wave signal is received, such received signal is directly converted into an intermediate frequency signal of 455 KHz, and thus is commonly referred to as a single super. More particularly, in the prior art radio receiver, during short wave reception, an input high frequency signal is applied through a tuning circuit to the base of a transistor forming a first mixer. The base of this transistor is also supplied with a local oscillation signal from the transformer of a first oscillator. Thus, the transistor carries out a first stage of frequency conversion and produces a first intermediate frequency signal at its collector. When variable capacitors are adjusted to provide the first intermediate frequency signal with a frequency of, for example, 10.7 MHz, this first intermediate frequency signal is derived through a tuning circuit, amplified by, for example, a transistor, and then applied through a tuning circuit to, for example, the base of a transistor forming a second mixer. The base of the second mixer transistor is also supplied with a power source voltage +V.sub.cc which is also applied to a second local oscillator. Thus, the second local oscillator oscillates to produce a local oscillation signal which is applied to the emitter of the last mentioned or second mixer transistor so that the latter carries out a second frequency conversion and produces a second intermediate frequency signal at its collector. When the local oscillation frequency of the second local oscillator is adjusted to provide the second intermediate frequency signal with a frequency of, for example, 455 KHz, this signal is derived through a tuning circuit and an intermediate frequency signal amplifier. Thus, a desired short wave signal can be received.
During medium wave reception with the prior art radio receiver, a received high frequency signal is supplied through a tuning circuit provided only for medium wave reception to the base of the transistor forming the second mixer. The base of this second mixer transistor is also supplied with the local oscillation signal from the transformer of the first local oscillator and with an automatic gain control voltage E.sub.A. However, the power source voltage +V.sub.cc is not applied to the second local oscillator during medium wave reception, and hence the second local oscillator does not oscillate. Accordingly, the received high frequency signal is converted by the second mixer transistor to an intermediate frequency signal with the local oscillation signal from the transformer of the first local oscillator. When the variable capacitor is adjusted to provide the converted or intermediate frequency signal with a frequency of, for example, 455 KHz, this signal is derived through the tuning circuit and intermediate frequency signal amplifier. Thus, a desired channel of medium waves or medium wave signal can be received.
In the above described prior art receiver, in order to select short wave or medium wave reception, a plurality of switches are changed-over preferably in ganged relation, so that such switches are desirably located close to each other. As a result, upon short wave reception, due to a stray capacity between switches of the first local oscillator and a switch connected to the base of the transistor forming the second mixer, the local oscillation signal from the first local oscillator is jumped in the base of the transistor forming the second mixer, or the local oscillation signal from the first local oscillator is induced in the bar-antenna tuning circuit for medium wave reception, and then jumped in the base of the second mixer transistor. By way of example, if it is assumed that the local oscillation frequency of the first local oscillator is f.sub.OA, that the local oscillation frequency of the second local oscillator is f.sub.OB, and the intermediate frequency from the second stage is f.sub.IB (for example, 455 KHz), as above, a self-spurious, whose frequency is nf.sub.OA .about. mf.sub.OB = f.sub.IB (in which n and m are positive integers), appears at the collector of the transistor forming the second mixer. This self-spurious is amplified by the intermediate frequency signal amplifier through its tuning circuit and mixed with the intermediate frequency signal of a desired channel. The foregoing results from the fact that the transistor forming the second mixer is used for both the short wave reception and the medium wave reception.